Lower alcohols (i.e. C 1-4 alcohols) are important components as reactants in a wide range of chemical processes and as a fuel source. Ethanol has been mandated by the United States government as a gasoline additive and/or as a major component of automobile fuel. The use of ethanol as a fuel additive has been gaining popularity because it is renewable and is a cleaner burning fuel source than most components of gasoline. Particularly, production of ethanol worldwide has been steadily increasing over the past years. Ethanol is usually produced by a fermentation process. The fermentation broth, typically containing 5 wt. % to 13 wt. % ethanol, is distilled to increase the ethanol content beyond 90 wt. % and requires purification to greater than 99 wt. % to be useful as fuel grade alcohol.
Due to the existence of the ethanol-water azeotrope at a concentration of about 95 wt. % ethanol, further ethanol concentration is accomplished using azeotropic distillation or by adsorption separation process such as a pressure swing adsorption (pressure swing adsorption) process. The dry ethanol stream preferably has less than 0.5% moisture content to meet the criteria for blending with gasoline. Azeotropic distillation requires the use of benzene to break the azeotrope. Because of the carcinogenic nature of benzene, other efficient separation techniques are desirable.
Pressure Swing Adsorption (PSA) is a separation process for selectively separating one component (“target component”) from of a liquid mixture. The target component is selectively adsorbed onto a solid adsorbent under relatively high pressure. At that pressure the other components are not adsorbed or is weakly adsorbed onto the solid adsorbent. After the capacity of the adsorbent to adsorb the target component is exhausted, the adsorbent is regenerated. Regeneration occurs by reducing the partial pressure of the target component in the adsorbent bed. This is accomplished by lowering the total pressure of the vapor in the adsorption bed and/or by passing a purge gas over the solid adsorbent. The target component is released by this combination of pressure reduction and purge from the solid adsorbent into the purge stream. The adsorbent bed is then re-pressurized and has a regenerated capacity to adsorb more of the target component onto the surface of the solid adsorbent.
The original pressure swing adsorption cycle was invented by Skarstrom in 1960 (See U.S. Pat. No. 2,944,627). According to Skarstrom, the two steps of adsorption and regeneration (or purge step) are carried out in two adsorbent beds operated in tandem, enabling the processing of a continuous feed. Since the introduction of the Skarstrom cycle, many more sophisticated pressure swing adsorption processes have been developed and commercialized. Such processes have attracted increasing interest more recently because of their low energy requirements and low capital investment costs.
One of the earliest disclosures of removing water (target component) from ethanol by pressure swing adsorption is found in U.S. Pat. No. 2,137,605 (“Derr”). Derr describes a method that uses freshly reactivated alumina to adsorb the moisture.
U.S. Pat. No. 4,465,875 (“Greenbank”) and U.S. Pat. No. 4,407,662 (“Cinder”) describe the use of molecular sieves to dry the ethanol. U.S. Pat. No. 4,273,621 (“Fornoff”) describes a process for the ethanol dehydration in the presence of carbon dioxide using a crystalline zeolite having a pore size of 3 Angstroms with high affinity for water. The 3 angstrom pore size is highly selective because the binding site for water is within a pore that is large enough to permit water to enter into the pore, but is too small to allow ethanol to enter the pore.
United States Patent Application 20070000769 (“Brown I”) incorporated by reference in its entirety discloses a process for producing fuel grade alcohol from a fermentation process that includes use of pressure swing adsorption. PCT Publication No. WO 2010/096626 (“Brown H”) discloses the use of pressure swing adsorption in a process for recovery of methanol.
Common feature for all pressure swing adsorption ethanol dehydration cycles encountered in the industry today is the low value of the ratio of the purge time to the adsorption time—often about 0.05 or less. The other previously unsolved constraint of the current-state-of-the-art is that the rate of the blowdown and the pressurization step is limited to ˜25 Psia/min. A faster rate will make the bed particles fluidize and thus cause irreversible damage to the adsorbent material.
Thus, there is a need for a system of operation of an adsorption bed for separation of water from lower alcohols that has a greater volume output without compromising purity, an improvement in purity without compromising output, a reduction in the blowdown or pressurization time without increasing the loss of adsorbent. The present invention addresses these and other needs.